A telephoto type of zoom lens configuration having five or more lens groups, in which the power of the first lens group is positive and the power of the rearmost lens group is negative, is often used as a so-called ultra telescopic zoom lens system, in which the focal length at the long focal length extremity exceeds 400 mm (converted to 35 mm format). In order to be applied for autofocusing, this type of ultra telescopic zoom lens system often uses, as a focusing method for focusing on an object at infinity to an object at a close distance, an internal focusing method in which the lens groups between the first lens group and the rearmost lens group are moved in the optical axis direction thereof, or often uses a rear-focusing method in which the rearmost lens group is moved in the optical axis direction.
In a telephoto zoom lens system, since axial chromatic aberration and lateral chromatic aberration become large at the long focal length extremity, there is a requirement for such aberrations to be favorably corrected. Furthermore, telephoto zoom lens systems are sometimes required to have specifications in which the f-number is faster than 6.0 at the long focal length extremity; accordingly, spherical aberration is also required to be favorably corrected. However, in a so-called ultra telephoto zoom lens system in which the focal length at the long focal length extremity exceeds 400 mm, there is a tendency for axial chromatic aberration, lateral chromatic aberration and spherical aberration to increase, thereby deteriorating the optical quality. Furthermore, in the case where a telephoto zoom lens system is used in an interchangeable lens camera of a digital SLR camera, since the lens diameter of the rear lens group cannot be enlarged due to mount restrictions, the quantity of peripheral light at the long focal length extremity decreases.
Patent Literature No. 1 discloses a zoom lens system having a configuration of six lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a negative lens group, a positive lens group and a negative lens group. The zoom lens system of Patent Literature No. 1 utilizes a floating focusing system in which five lens groups (a first lens group, a third lens group, a fourth lens group, a fifth lens group and a sixth lens group) are moved in the optical axis direction during zooming, and two lens groups (the fourth lens group and the sixth lens group) are moved in the optical axis direction during focusing. Accordingly, the entire mechanism, which includes a zooming mechanism and a focusing mechanism, becomes enlarged, has an increased weight and increased complexity. Furthermore, in the zoom lens system of Patent Literature No. 1, a large amount of spherical aberration and field curvature occur during focusing, thereby deteriorating the optical quality.
Patent Literature No. 2 discloses a zoom lens system having a configuration of five lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a positive lens group and a negative lens group, in which all five lens groups move in the optical axis direction during zooming, and a zoom lens system having a configuration of six lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a positive lens group, a positive lens group and a negative lens group, in which all six lens groups move in the optical axis direction during zooming. However, in the zoom lens system of Patent Literature No. 2, the entire mechanism including a zooming mechanism becomes enlarged, has an increased weight, increased complexity, and a large amount of spherical aberration occurs at the long focal length extremity, thereby deteriorating the optical quality.
Patent Literature No. 3 discloses a zoom lens system configured of five lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a positive lens group and a negative lens group, and in which a diaphragm is moved independently of each lens group in the optical axis direction. In the zoom lens system of Patent Literature No. 3, the focal length at the long focal length extremity is 490 mm (which is long), however, the f-number is 6.25 (which is slow) at the long focal length extremity, and a large amount of axial chromatic aberration occurs at the long focal length extremity, thereby deteriorating the optical quality. Furthermore, in the zoom lens system of Patent Literature No. 3, the second lens group is stationary relative to the image plane during zooming, however, since the diaphragm is moved as an independent group in the optical axis direction, this results in five independent groups being moved in the optical axis direction, and the entire mechanism, including the zooming mechanism, becoming large, having an increased weight and increased complexity. Furthermore, although the zoom lens system of Patent Literature No. 3 utilizes a front-focusing method, in which the first lens group is moved in the optical axis direction during focusing, since the diameter and weight of the first lens group are large, this zoom lens system is not suitable for autofocusing.
Patent Literature No. 4 discloses a zoom lens system having five lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a positive lens group, and a negative lens group. Although the zoom lens system in Patent Literature No. 4 has a focal length of 485 mm (which is long) at the long focal length extremity and the f-number is 5.8 (which is somewhat fast), a large amount of chromatic aberration occurs at the long focal length extremity, thereby deteriorating the optical quality. Furthermore, although the zoom lens system of the Patent Literature No. 4 utilizes a rear-focusing method, in which the fifth lens group is moved in the optical axis direction during focusing, large amounts of spherical aberration and field curvature occur, thereby deteriorating the optical quality.